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Full text of "Text Book Of Mechanical Engineering"

614

Initial Condensation.

8.  The range of working temperature is small in comparison with
the temperatures themselves : ^ being fixed to prevent burning of
cylinder oils and packing, and r2 by the cold well temperature.

9.  Heat is lost by radiation.

10.  The substance is lost by leakage, taking heat with it.

n. Wherever imperfectly-resisted expansion occurs, reversibility
is impaired : e.g., the 'drop' into receiver in a compound engine.

12.  Various small losses, shewn on indicator diagram : e.g.,
wire drawing, &c.

13.  Work is lost in (a) the * solid' friction of the engine parts,
(b) the fluid friction of the passing steam.    (See App. HI., p. 935.)

Initial" Condensation and Re-evaporation.—When
hot saturated steam enters a cylinder cooled to exhaust tempera-
ture, an ' initial condensation' occurs, which is not immediately
apparent on the pressure diagrams. After cut-off, further con-
densation lowers the expansion curve, as shewn dotted at AB,
Fig. 618. But cylinder and steam becoming more equal in tem-
perature, the latent heat, liberated during liquefaction, is permitted
to raise the curve, as at B c, by causing a certain re-evaporation.
The first loss is, however, very great, and by no means made up
by the second gain, so there is always a quantity of water rejected
at release, some of which evaporates during exhaust and creates a
back pressure. These losses may be mitigated (i) by applying
clothing in quick running engines, and thus securing approximate
adiabatic expansion, (2) by adopting a steam jacket for engines
of a slower type, where there is time for the heat to be taken up,
or (3) by superheating the steam before admission, and partially
removing the first cause. The jacket both assists re-evaporation
at an earlier, and consequently more available, portion of the
stroke, and prevents to some extent initial condensation : the
experimental gain being stated at from 10 to 20 per cent. Lique-
faction in the jacket is not so detrimental, but in the cylinder the
water acts as a conductor from the steapito the metal. Live steam
should always be used for the jacket, and efficient drainage applied.

Theory of Compounding^—'Another way of decreasing
liquefaction is to divide the work among 2, 3^ or 4 cylinders; and,
if great differences of temperature be employed, no other course
is possible. Thus we arrive at the Compound, Triple, or Qn&d-